共查询到19条相似文献，搜索用时 182 毫秒
1.
利用埃及北部Faiyum盆地获得的高取芯率沉积物岩芯,进行沉积物多种磁性参数的测量,结合有机碳、介形虫、粒度等分析,在AMS14C加速器测年的基础上,建立全新世以来湖泊沉积物磁性特征变化的时间序列.结果表明,粒度效应以及沉积后的各种次生作用对沉积物的磁性特征没有明显的影响,磁性变化主要反映了沉积物不同来源组成的相对变化.全新世前沉积物磁性较弱,主要含不完全反铁磁性矿物,与周边沙漠的物质相似,结合其粒度特征,沉积物来源应以近源物质为主.全新世早中期(约10 5.4 ka BP)沉积物磁性变化相对稳定,有机质含量也较高,指示了来自尼罗河较为稳定的物质供应;而大约5.4 ka BP尤其最近约4.2 ka BP以来,磁性的明显变化反映了流域降水减少情况下,来自青尼河物质贡献的相对增加;最近约2.0 ka BP以来沉积物的磁性变化,则更多地与盆地人类活动的强化有关.总体而言,Faiyum盆地全新世以来的环境演变主要受控于全新世以来尼罗河与盆地的水力学联系.即:全新世前盆地未与尼罗河连通时,沉积物主要来源于周边沙漠的风成物质;而受全新世早-中期来流域季风降水增加的影响,泛滥的尼罗河为盆地提供了相对稳定的物质供应,湖泊也处于高湖面;全新世晚期以来,随着流域干旱化的加剧,尼罗河与盆地的连通性开始减弱,来自高磁性的青尼罗河物质贡献开始相对增加.最近约2.0 ka BP以来,虽然仍有人工运河连接尼罗河与盆地,但沉积物磁性的显著变化更多地反映了盆地人类活动的不断强化. 相似文献
2.
对苏北盆地兴化1#钻孔岩心沉积物进行了质量磁化率、频率磁化率、变化曲线、等温剩磁和磁滞回线等参数的测试分析，结果表明，粘土质沉积物中的主要载磁矿物为磁铁矿和赤铁矿；砂质沉积物中的磁性矿物除了磁铁矿和赤铁矿外，还含有少量的磁赤铁矿和针铁矿.针铁矿普遍存在于钻孔下部的样品中.整个钻孔沉积物中的磁性矿物颗粒都是介于单畴和多畴之间的准单畴颗粒，但粘土质沉积物中的磁性矿物颗粒更趋近于单畴颗粒，而砂质沉积物中的磁性矿物颗粒更趋近于多畴.根据不同的磁性矿物组合特征，选择合理的温度和磁场强度区间对古地磁退磁结果进行分析，得到沉积物可靠的特征剩磁方向，为古地磁年代学提供依据. 相似文献
3.
呼伦湖地区扎赉诺尔晚第四纪湖泊沉积物的磁化率变化及其影响因素 总被引：11，自引：1，他引：11
呼伦湖盆地扎赉诺尔晚第四纪湖泊沉积物为砂，泥质碎屑沉,地东露天矿剖面沉积物样品进行了低，高频磁化率测量，粒度分析，磁性矿物成分鉴定和Al2O3 ,Fe2O3含量分析，分析表明，磁铁矿是沉积物磁化率的主要贡献者，全铁含量控制了磁化率值的变化，磁化率与沉积物粒度呈负相关，进一步讨论认为，磁性矿物主要为粉砂－粘土级碎屑，集中分布在细粒沉积物中，因此造成其率较高，砂质沉积物中频率磁化率较高，其原因可能是其中的磁性颗粒比粉砂一粘土级沉积物中的磁性颗粒更细一些。 相似文献
4.
长江口潮滩沉积物的磁学性质及其与粒度的关系 总被引：43，自引：0，他引：43
对长江口潮滩沉积物的环境磁学研究表明, 沉积物磁性特征由多畴/假单畴磁铁矿所主导. 磁铁矿除陆源碎屑成因外, 在磁性最强的石洞口排污口附近沉积物中出现了球形磁性颗粒, 系工业污染的产物. 粒度是影响长江口潮滩沉积物磁性特征的重要因素, 磁化率、饱和等温剩磁与8~16 mm粒级含量呈正相关, 而非磁滞剩磁磁化率等指示细颗粒亚铁磁性矿物的参数与<4 mm乃至<32 mm粒级组分高度正相关. 这一特征表明, 在将磁化率作为污染物含量代用指标时, 需考虑粒度造成的磁化率波动的可能影响, 非磁滞剩磁磁化率则可以作为沉积物细粒级组分的代用指标, 并应用于污染物含量的粒度校正. 相似文献
5.
珠江三角洲沉积物的岩石磁学性质及其环境意义 总被引：1，自引：0，他引：1
选取珠江三角洲区域位于广州番禺的PD钻孔进行沉积物岩石磁学和环境磁学记录的研究. 岩石磁学实验表明钻孔大部分沉积物以低矫顽力的磁铁矿类矿物为主要控磁矿物, 但仍存在少量铁硫化物. 在有机质丰富的灰黑色黏土沉积中, 铁硫化物含量增加, 成为影响沉积物磁性特征的主要矿物, 标志着海退后富含有机质而缺氧的沼泽环境. 以赤铁矿和针铁矿类矿物为主要控磁矿物的花斑黏土, 表明了一个较长时期的暴露风化过程, 代表着沉积间断. 在以磁铁矿类矿物作为主要载磁矿物的沉积物中, 磁铁矿浓度及粒度的变化主导磁化率(κ)、饱和等温剩磁(SIRM)等环境磁学参数的变化, SIRM/κ 和χarm/SIRM的大小旋回响应于海平面的升-降过程, 它们的高值反映了磁性矿物粒度变细、海平面上升. 在沉积学分析的基础上, 根据环境磁学参数截然不同的变化规律, 结合微体古生物数据, 将珠江三角洲沉积物代表的环境分为两次显著的海平面上升－下降旋回, 晚更新世和全新世的海侵又包含若干个比较明显的次级海平面波动过程. 相似文献
6.
大陆架作为海陆相互作用的关键地区,对于研究大陆的构造演化、海陆变迁、海平面升降以及气候变化具有重要意义.然而由于不同研究方法的局限性,目前对大陆架沉积物年代学及其蕴含地质信息的认识仍然不足.南海是西太平洋最大的边缘海,是全球海洋沉积作用最为活跃的地区之一,也是海陆相互作用最为典型的区域.作为东亚大陆物质的主要沉积区,南海已经受到了学术界越来越多的关注.然而,目前的研究工作主要集中于沉积连续、信号记录稳定但沉积速率较慢、总体分辨率较低的深海区沉积物.相对而言,沉积速率较快、分辨率较高的浅海大陆架沉积为高分辨率年代学和古环境的研究提供了重要的地质材料,但由于大陆架沉积环境动荡导致沉积信号记录不稳定甚至缺失.针对南海大陆架沉积,尤其是对钻孔沉积物高分辨率年代学研究仍相对较少,限制了对南海构造与气候演化过程的认识.为了更好地限定南海北部陆架区更新世晚期沉积物的年代,研究其中蕴含的古环境信息,探讨东亚地区气候变化的驱动机制问题,同时为南海海域活动构造研究提供年代学框架,文中以南海北部DG钻孔为研究对象,在微体古生物化石和碳同位素年龄(14C)数据的基础上,利用大陆架沉积物磁化率与深海氧同位素的对比对其沉积物年代学进行了系统研究.基于此,结合色度和孢粉结果,对其古气候意义进行了初步探讨.结果表明,该钻孔沉积物的磁化率可对应于深海氧同位素的阶段1—阶段9(MIS 1—MIS 9),底部年龄约为300ka,磁化率低值区间对应于冰期,高值区间对应于间冰期.这与该钻孔沉积物中的孢粉和色度所记录的古环境信息相吻合.冰期时气候较为寒冷,水体变浅,沉积物搬运距离相对增大,矿物以氧化作用为主,主要形成弱磁性的磁性矿物(如赤铁矿),导致磁化率较低;间冰期时,气候相对暖湿,水体变深,沉积物搬运距离相对缩短,矿物以还原作用为主,主要形成强磁性的磁性矿物(如磁铁矿等),导致沉积物的磁化率显著增强.因此,南海大陆架北部更新世晚期沉积物的磁化率变化可以反映东亚地区更新世晚期以来冰期—间冰期气候旋回.磁化率与深海氧同位素的对比作为一种晚第四纪松散沉积物的相对定年方法,在南海北部陆架区更新世晚期沉积物定年方面是适用且可靠的,可为海洋大陆架沉积物定年和对比研究提供新的参考. 相似文献
7.
以热磁分析为主,对中国北方泥河湾盆地更新世河湖相地层中灰绿色粉砂和灰黄色粉砂/细砂两种典型沉积物进行了详细的岩石磁学研究,有效确定了这两类沉积物中磁性矿物的种类、粒度特征以及加热过程中磁性矿物的变化过程和产物,并对其包含的古环境意义进行了初步探讨.灰绿色粉砂样品主要含有磁铁矿和赤铁矿两种磁性矿物,磁性相对较弱,颗粒相对较细;灰黄色粉砂/细砂主要含有磁铁矿和磁赤铁矿,磁性相对较强,颗粒相对较粗.在氩气环境中经700℃加热处理后,这两种沉积物中的绿泥石都分解,并生成超细粒（处于超顺磁和单畴颗粒区间）的磁铁矿,导致磁化率大幅升高.因此泥河湾盆地沉积物的热磁特征可以用来检测样品中绿泥石的相对含量,进而反映该地区化学风化作用强度的变化.此外,灰绿色粉砂样品中绿泥石含量比灰黄色粉砂/细砂样品的含量高,在氩气中加热后,磁化率升高幅度也较高,可能反映了化学风化相对较弱的沉积环境. 相似文献
8.
湖泊沉积物中胶黄铁矿的鉴出及其磁学意义 总被引：6，自引：1，他引：6
对青藏高原东部若尔盖盆地钻孔获取的湖泊沉积物进行了详细的岩石磁学调查及矿物学鉴定, 发现大量胶黄铁矿存在, 并认定其为主要的磁载体. 该矿物颗粒细小, 粒度均匀, 可能为生物化学成因. 与大量文献报道的相反, 在长期暴露于空气后, 样品中的胶黄铁矿仍未被氧化. 这可能与钻孔内该矿物被硅质胶结物包裹有关. 研究结果初步揭示了湖泊沉积物磁学研究的复杂性, 同时也为环境磁学机理的研究提供了新的线索. 相似文献
9.
洪湖沉积物的磁性特征及其环境意义 总被引：9，自引：0，他引：9
以洪湖沉积物为研究对象,通过对Hh01-2(B)孔岩芯磁参数的测量发现在岩芯所代表的时间段内洪湖沉积物磁化率呈现了明显的四个主阶段变化;频率磁化率变化与磁化率变化具有较好的相关性,亚铁磁性矿物的变化主导了沉积物磁化率的变化.同时结合年代、孢粉分析以及参考相关历史记载资料,发现磁化率高值段对应湖水动力强、湖面较宽、气候较湿润,而磁化率低值段对应湖水动力弱、湖面较小、气候偏干旱.从而得出了洪湖沉积物磁参数曲线波峰和波谷分别对应洪湖在历史上曾经历了扩张和收缩、气候温湿和冷干的旋回. 相似文献
10.
对尼罗河三角洲平原东北部一个古埃及文化遗址(Kom El-Khilgan)1.5 m长的沉积物剖面KH1开展14C测年、沉积物粒度和孢粉分析.根据岩性、粒度和测年等结果综合判定:剖面150~100 cm地层为早全新世河流相沉积;100~27 cm地层为早、中全新世(8000—4000 cal a BP)三角洲冲积平原沉积,其中涵盖了古埃及前王朝至古王国时期(6500—4131cal a BP)的人类活动信息;27~0 cm为现代表土层.孢粉分析结果反映了在距今8000~4000 a期间:1)研究区气候由湿润过渡为温和,最后趋于干旱;2)随着气候变干,尼罗河上游水文变化对三角洲环境影响减小,研究区由沼泽湿地逐渐过渡为干草甸环境,更多的土地可以被人类利用;3)常被用来指示农业活动的35μm禾本科、陶器碎片和炭屑的出现并逐渐增加,表明该遗址附近的人类农业活动始于距今6500 a左右,并持续至距今4000 a前后.上述证据表明尼罗河三角洲全新世早—中期湿润向干旱过渡的气候驱动了人类农业活动的开展.本研究为进一步探讨尼罗河三角洲古文明对环境适应性研究提供了科学依据. 相似文献
11.
Mohamed A. Gamal 《Journal of Seismology》2009,13(1):13-30
In order to determine the fundamental period of soil vibrations in Cairo, 174 microtremors stations, in conjunction with mobile
accelerographs, were used. The result was a collection of long-period microtremors and ambient noise arising from cultural
disturbances. The Nile Valley shows some fixed peaks at 2.5–3.5 Hz at the center of the basin, while the Nile’s surrounding
area shows a fundamental peak of 4–5 Hz, leaving a 5–7-Hz resonance peak for the sand-like, gravelly soil from Abbasiya to
the airport. A frequency-dependent soil amplification map is drawn, which includes seismic microzonation maps for Cairo. Based
on the above, a maximum acceleration map for two important earthquakes affecting Egypt in the last century is produced (Faiyum,
1992, and Aqaba, 1995). 相似文献
12.
General information on the Nile River basin is presented. Specific features of the morphological regime of the Nile Delta and its near-shore zone, as well as the delta water balance and groundwater regime are discussed. The dynamics of the sea coastline of the Nile Delta is described in great detail. It is shown that the recent delta of the Nile and its sea coastline have been formed under the joint impact of the eustatic sea level rise, subsidence of sediments in the northern part of the deltaic plain, and considerable decrease in the river sediment runoff after the construction of the Aswan High Dam. 相似文献
13.
— The Nile valley and the Nile delta are part of the active rift that is probably connected with the Red Sea tectonism. This zone is characterized by small-to-moderate size earthquakes that have caused extremely severe damage to recent and historical constructions. The most vulnerable area along the Nile valley is the one of Cairo-Faiyoum. Small local and large distant earthquakes could be a source of huge socio-economic damage in this area. The loose soft alluvial sediments of the Nile Canyon are the main factor behind this potential damage because they may greatly amplify the ground motion, as demonstrated by strong ground motion modelling. The largest amplification is generally concentrated along the edges of the graben and occurs at frequencies between 1 Hz and 2 Hz. This may explain the huge damage caused by distant earthquakes during recent and historical times. The distribution of intensity values during the events of 1926 and 1992 is well correlated with the modelled spatial distribution of the spectral amplification.Dr. El-Sayed passed away in Mansoura on February 11, 2003. 相似文献
14.
Ethiopia and the Eastern Nile Basin 总被引：1，自引：0，他引：1
15.
Barak Herut Hava Hornung Michael D. Krom Nurit Kress Yuval Cohen 《Marine pollution bulletin》1993,26(12):675-682
Concentrations of mercury, lead, copper, zinc, cadmium, iron and aluminium were recorded in coastal and river mouths surficial sediments, collected along the Mediterranean coast of Israel, during 1988–1991. The levels of trace metal concentrations when normalized against Fe concentration (trace metal/iron ratios) indicated no contamination at most of the stations along the coast. Enrichment of Hg, Cu, Zn and Cd attributed to land-based point sources of pollution, was found in the estuaries of the Kishon and Yarkon rivers. The normalized Pb, Cu and Zn data showed a trend of increasing values from the south toward the north. This S-N trend could be a result of three processes which may be occurring simultaneously: progressive mixing between sediment with a low trace metal/iron ratio in the south, which probably represents a single source derived from the Nile region with (a) non-point source input of pollutants by atmospheric transport, or (b) with point sources of pollutants introduced by rivers and streams, or (c) a natural change in the mineralogy of the sediments along the coast with a relative increase in the trace metals or a decrease in iron as one progresses further north. The relative importance of these three possible processes remains to be established. 相似文献
16.
Eduardo Garzanti Sergio Andò Giovanni Vezzoli 《Earth and Planetary Science Letters》2009,277(3-4):422-432
This article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition, which cause systematic distortion in quantitative provenance analysis (“environmental bias”). Traditional procedures aimed at eliminating textural noise find limited success. The Gazzi–Dickinson method cannot remove hydrodynamic-related modal variability. Multiple size-window strategies are time-consuming. Narrow or moving size-window strategies represent misleading or impractical short cuts, being less convenient options than simply analysing each sample in bulk. New concepts introduced here unravel the superposed causes of compositional variability in modern sediments. Intrasample modal variability, fundamentally explained by settling-equivalence relationships, can be accurately modelled mathematically. Intersample modal variability, principally resulting from selective entrainment, can be assessed and removed by a simple principle. In absence of provenance changes and environmental bias, the weighted average density of terrigenous grains (SRD index) should be equal, for each sample and each grain-size class of each sample, to the weighted average density of source rocks. By correcting relative abundances of detrital minerals in proportion to their densities, we can restore the appropriate SRD index for any provenance and subprovenance type in each sample or grain-size class. Modal variability is effectively reduced by this procedure, which can be applied confidently to modern sediments deposited by tractive currents in any environment. Good results are obtained even for placer sands and finest classes where heavy-mineral concentration is strongest. Such “SRD correction” also successfully compensates for biased narrow-window modes, thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology. After compensating for settling-equivalence and selective-entrainment effects, residual size-dependent compositional variability may be provenance-related. Minor in Ganga–Brahmaputra sediments, provenance-related effects are spectacularly displayed in the Nile basin, where volcaniclastic silt mixes with basement-derived quartzofeldspathic sand and wind-blown Saharan quartz. 相似文献
17.
On 12 October 1992, an earthquake, magnitude mb = 5.9 and M s = 5.2, hit the City of Cairo, Egypt. It was this century's largest earthquake in northern Egypt with related destruction in the City of Cairo, the Nile Valley and the Nile Delta areas. Our source parameter determinations show that the 1992 earthquake had a normal faulting mechanism, seismic moment M o = 5.2 × 1017 Nm, centroid depth of 23 km and a source time function duration of 3 seconds. The mechanism is compared with those corresponding to two other events that occurred in the northern Red Sea. The similarity between the mechanisms as well as the spatial distribution of the geological faults around Cairo suggest seismic activity along the extension of the stress field of the Red Sea rift system to the area around the City of Cairo. This situation affects the level of seismic hazard in the Cairo area. The 1992 earthquake belongs to an unusual class of relatively small, M w > 6.0, yet destructive earthquakes. The damage caused by these events is usually attributed to their shallow focal depth, 5 km, and to amplification of seismic waves in the local soil beneath the damaged structures. However, the Cairo earthquake deviates from other events of this class since the focal depth was determined to be 23 km. We calculated synthetic accelerograms for the 1992 earthquake with the loose sediments observed in the Nile Valley, and show that this enhanced the amplitude of the acceleration by a factor of two. However, the determined accelerations, about 0.5 m s-2, cannot alone explain the relatively large amount of damaged structures. Hence, a major cause to the destruction is likely the poor state of construction of the Cairo buildings. 相似文献
18.
19.
The dry wadis ‘ephemeral channel’ constituting the main tributaries to the Nile River in Kom Ombo are structurally and tectonically controlled and exhibit complex drainage pattern. This complicated drainage pattern is inherited from the morpho‐tectonic evolution of the ancestral Nile River (‘Protonile’), which drained the Eastern Desert during the Middle Pleistocene. A digital elevation model derived from Shuttle Radar Topography Mission data is used to delineate the contemporary drainage networks and their catchments. Satellite images acquired during a flash flood event were used to validate the delineated watershed divides and flow pathways, particularly where the courses of dry wadis are interlocked. Currently, the westward flow of Wadi Abu‐Suberah is derived from a small area in the Eastern Desert, as the palaeo‐upper reaches of this wadi were captured due to tectonic movements along NW/SE and N/S faults by wadis in the Kharit and Elewa areas. The influence of these tectonic movements on groundwater distribution is also shown where the deep Nubian aquifer discharges its water into the Quaternary aquifer through fault planes. The northward flowing ‘Protonile’ main course has tectonically shifted from the Gallaba plain in the Western Desert, eastward to the current Nile River course. This shift has produced several cut‐off segments of the palaeo‐tributary drainage that was originally flowing westward towards the old ‘Protonile’ main course in the Gallaba plain. However, these segments have reversed their surface run‐off flow directions eastward towards the current Nile course; they could include potential groundwater resources, as their alluvium may be still recharged by the Nile River. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献